GuideTransformers12 min read
Transformer oil and dissolved gas analysis
Transformer oil insulates and cools a liquid-filled transformer, and it also records what happens inside the tank. An oil sample answers three questions: does the oil still insulate (dielectric strength, moisture, acidity), is it aging (interfacial tension, color), and has the transformer been faulting (dissolved gas analysis)? DGA is the most useful single test on a used transformer, because overheating, partial discharge and arcing each leave their own gases behind.
The short version
- Mineral oil fills most units; natural ester (FR3) and silicone are the less-flammable fluids allowed indoors; askarel (PCB) fluid was banned in 1979 and old units are tested for it.
- The oil screen, with IEEE C57.106’s limits for units up to 69 kV: breakdown voltage at least 26 kV (ASTM D877), moisture under 35 ppm, acidity under 0.2 mg KOH/g, interfacial tension over 25 mN/m.
- DGA, which gas means what: hydrogen for partial discharge; methane and ethane for mild overheating; ethylene for a serious hot spot; acetylene for arcing; carbon monoxide and dioxide for overheated paper.
- Typical limits for a healthy unit (IEEE C57.104’s widely used table): hydrogen 100 ppm, methane 120, ethane 65, ethylene 50, acetylene 1, carbon monoxide 350, carbon dioxide 2,500; total combustible gas 720.
- The trend matters as much as the level: a second sample a few months on shows whether a fault is active.
- On a used transformer ask for a DGA and oil screen dated within six months, the sample’s oil temperature, and a PCB result on anything built before the mid-1980s.
What the oil does
A liquid-filled transformer’s windings sit in a tank of oil. The oil does two jobs: it insulates, filling every gap between windings, turns and tank with a fluid that withstands far more voltage than air; and it cools, carrying heat from the windings to the tank walls and radiators by convection (ONAN on the nameplate: oil natural, air natural) or by pumps and fans (ONAF, OFAF).
It does a third job by accident. The oil is in contact with everything inside the tank, and when something overheats or arcs, the oil and the paper insulation break down into gases that dissolve in it. Draw a sample and the oil tells you what has been going on, and roughly where and how hot. That’s dissolved gas analysis, and it’s why an oil sample is the first thing a buyer asks for on a used transformer.
Kinds of insulating fluid
| Fluid | Fire point | Where it’s used | Notes |
|---|---|---|---|
| Mineral oil (ASTM D3487, Type II) | About 160 °C | Most pad-mounts, pole-mounts and substations | Cheapest; well understood; needs containment and, indoors, a vault |
| Natural ester (Envirotemp FR3, and others) | Over 300 °C | New pad-mounts, indoor units, data centers, anywhere fire or spills matter | Less-flammable and biodegradable; absorbs more water than mineral oil; nameplate cooling class K (KNAN) |
| Synthetic ester (Midel 7131) | Over 300 °C | Indoor and traction units, mostly outside North America | As natural ester, more stable in open-breathing units |
| Silicone | Over 300 °C | Indoor units from the 1980s and 90s | Less-flammable; not biodegradable; can’t be mixed with other fluids; a silicone unit stays silicone |
| High-molecular-weight hydrocarbon (R-Temp, Beta Fluid) | Over 300 °C | Indoor units, older less-flammable installations | A heavy mineral oil; less-flammable |
| Askarel (PCB) | Non-flammable | Indoor units built before 1979 | Banned; see PCBs below |
A listing names the fluid, or should. Where a seller says only “oil”, it almost always means mineral oil; a nameplate with KNAN or KNAF cooling, or an FR3 label by the fill valve, means natural ester; a 1980s indoor unit marked “silicone” or a brand like R-Temp is what it says. The fluid matters to where the unit can go (an FR3 unit may be allowed indoors or near a building where a mineral-oil one isn’t) and to the limits the tests are judged against, which differ a little for esters (IEEE C57.147).
The oil quality tests (the oil screen)
The screen is a set of physical and chemical tests on the oil itself. Together they say whether the oil still insulates and cools, and how far along it is in its life.
| Test | Method | What it shows | In-service limit |
|---|---|---|---|
| Dielectric breakdown voltage | ASTM D877 (2 mm gap) or D1816 (1 mm gap) | How much voltage the oil withstands. Water and particles bring it down. | At least 26 kV (D877); at least 23 kV (D1816, 1 mm) |
| Moisture (Karl Fischer) | ASTM D1533 | Water dissolved in the oil, in ppm. The paper holds many times more; a warm sample shows more water than a cold one from the same unit. | Under 35 ppm |
| Acidity (neutralization number) | ASTM D974 | Acids from oxidation of the oil, which attack the paper and lead to sludge. | Under 0.2 mg KOH/g |
| Interfacial tension | ASTM D971 | The tension between the oil and water; falls as polar contaminants build. The earliest sign of oxidation. | Over 25 mN/m (dynes/cm) |
| Color | ASTM D1500 | Darkens with age and oxidation. A quick indicator, not a limit by itself. | Under 3.5 |
| Power factor (dissipation factor) at 25 °C | ASTM D924 | Loss in the oil as an insulator; rises with contamination and polar compounds. | Under 0.5% |
Rounded from IEEE C57.106’s in-service tables. New oil tests far better (breakdown over 30 kV, moisture under 10 ppm, acidity under 0.03). Higher-voltage classes have tighter limits; ester fluids have their own (IEEE C57.147).
The screen tells you about the oil, and oil can be fixed: filtered, dried, reclaimed or replaced. What it hints at but can’t measure is the paper. Moisture in the oil in equilibrium with the paper means the paper is wet, and paper that has been wet and hot for years has lost strength it can’t get back. That’s why the moisture figure comes with the oil temperature, and why a high reading on a used unit is a reason to ask how it was stored, not just to plan a dry-out.
Dissolved gas analysis: what each gas means
A fault inside the tank heats or arcs through the oil and the paper, breaking them into gases that dissolve in the oil. The lab extracts the gases from a sample (ASTM D3612) and measures each one in parts per million. Which gases appear depends on how hot the fault is and whether paper is involved.
| Gas | Made by | Points to |
|---|---|---|
| Hydrogen (H₂) | Partial discharge (corona) in the oil, at low energy; also produced in small amounts by every other fault | Partial discharge: voids, bubbles, a sharp edge, wet insulation |
| Methane (CH₄) and ethane (C₂H₆) | Oil overheated at low temperature, about 150 to 300 °C | A warm spot: a loose connection, a tap changer contact, circulating currents in the core clamps |
| Ethylene (C₂H₄) | Oil overheated above about 300 °C | A serious hot spot: an overloaded lead, a bad joint, core heating, a blocked cooling duct |
| Acetylene (C₂H₂) | Arcing, above about 700 °C | Arcing: a connection breaking under load, flashover, a tap changer fault. The gas to worry about |
| Carbon monoxide (CO) and carbon dioxide (CO₂) | Overheated paper (cellulose) | Winding insulation aging or a hot spot in the windings, rather than in the oil or the core |
| Oxygen (O₂) and nitrogen (N₂) | Air | A leak in a sealed unit, or a free-breathing (conservator) design; high oxygen speeds oxidation |
The pattern is what matters. A unit making methane and ethane with a little ethylene has a hot spot in the oil, likely a connection; one making carbon monoxide along with them has the hot spot in the windings; one making acetylene is arcing, whatever else it shows. Hydrogen on its own, in a unit otherwise clean, is partial discharge, often from moisture, and worth a second sample.
Reading a DGA report
A report lists each gas in ppm, usually with the total dissolved combustible gas (TDCG: everything but oxygen, nitrogen and carbon dioxide) and a rating of the unit’s condition. The ratings most labs still use come from the 2008 edition of IEEE C57.104, which sorted transformers into four conditions by the level of each gas. Condition 1 is a healthy unit:
| H₂ | CH₄ | C₂H₆ | C₂H₄ | C₂H₂ | CO | CO₂ | TDCG |
|---|---|---|---|---|---|---|---|
| 100 | 120 | 65 | 50 | 1 | 350 | 2,500 | 720 |
The 2019 edition replaced these with percentile tables that depend on the transformer’s age and oxygen content, and put more weight on the rate of change. Labs commonly report against both.
Three things to take from a report:
- The levels
- Any gas well over its Condition 1 limit says there has been a fault. Acetylene over a couple of ppm, or ethylene over about 100 ppm, deserves an explanation before purchase.
- The rate of change
- A unit with 200 ppm of hydrogen that has read 200 ppm for five years is stable; one that has gone from 20 to 200 in six months has an active fault. One sample gives a level; two, months apart, give a rate. On a used unit, ask for the history.
- The ratios
- The Rogers ratios and the Duval triangle use the proportions of methane, ethylene and acetylene (and hydrogen and ethane) to classify the fault type: thermal below 300 °C, thermal above 700 °C, low- or high-energy discharge. Labs include a Duval triangle on most reports; it’s a quick way to see what kind of fault, if any, the gases describe.
Taking a sample
The sample is drawn from the bottom drain or sample valve into a glass syringe (for DGA, so no air touches the oil) and a bottle (for the screen), after flushing the valve clear, and sent to the lab within days, labeled with the unit, the date, the oil temperature and the ambient temperature. A sample pulled without care, with air bubbles or from a dirty valve, gives a bad result, so most buyers have the seller’s crew or a test crew take it rather than the seller’s forklift driver.
A sample can be taken with the transformer energized, which is what makes DGA the one test that doesn’t cost an outage. The lab work runs about $100 to $200 for an oil screen and a DGA together, and results take a week.
PCBs in older transformers
Until 1979, transformers meant for indoor use were often filled with askarel, a fluid of polychlorinated biphenyls (PCBs) that doesn’t burn. PCBs were banned that year, and mineral-oil transformers from the same era can carry traces of them from shared pumps and hoses at the factory or the service shop. The EPA regulates them under 40 CFR 761 by concentration:
| PCB content | Class | What it means |
|---|---|---|
| Under 50 ppm | Non-PCB | No restrictions; most units, and everything built since the mid-1980s |
| 50 to 499 ppm | PCB-contaminated | May stay in service; disposal of the oil and the unit is regulated |
| 500 ppm and over | PCB transformer | Use is restricted and registered; retrofilling or disposal under EPA rules; not something to buy |
A nameplate or a label will say “non-PCB” or “no PCBs” on anything built since about 1980, and sellers of older units usually have a test certificate (the listings here quote them where sellers give them: “certified under 1 ppm”). For an old unit without one, the test is cheap (ASTM D4059). A PCB-contaminated unit is legal to run but a liability to dispose of; the price should reflect it, and the seller should say.
What to ask for on a used transformer
- A DGA and an oil screen dated within six months, from a named lab, with the oil temperature at sampling. Older results too, if the seller has them, for the trend.
- The fluid type, from the nameplate, and the gallons, which the listing should give (it sets the containment and the freight).
- A PCB result on anything built before about 1985, or without a non-PCB label.
- How it was stored. A unit stored outdoors with its bushings off, or with a cracked gauge, has taken on water; the moisture result will say so, and the price should.
- What was done about any bad result. A seller who filtered the oil after a high moisture reading has treated the symptom; ask whether the unit was dried.
Our own process samples every liquid-filled unit before it ships, for the screen and a DGA, alongside the electrical tests in the testing guide, and the report goes to the buyer before the sale closes. Browse the pad-mount listings for the units in stock, or ask Magic Search for a unit with the fluid you need.
Questions people ask
What does dissolved gas analysis detect?
Faults inside a liquid-filled transformer that no external test can see: a hot connection, a hot spot in the core or windings, partial discharge in the insulation, and arcing. Each kind of fault breaks down the oil and the paper into a different mix of gases, which dissolve in the oil. A lab extracts the gases from a sample and measures hydrogen, methane, ethane, ethylene, acetylene, carbon monoxide and carbon dioxide in parts per million, and the mix and the amounts say what kind of fault, how hot, and whether it’s getting worse.
How often should transformer oil be tested?
Once a year for a transformer in service, with an oil screen and a DGA together, and more often (quarterly, or monthly) after a result that shows a fault developing. A used transformer should have both tests done before purchase, dated within the last six months, and a baseline sample taken when it’s energized at its new site.
What does acetylene in transformer oil mean?
Arcing: an electrical discharge hot enough (above about 700 °C) to make acetylene from the oil. A healthy transformer has none; the usual limit is 1 to 2 ppm. A rising acetylene level means an active arcing fault, such as a loose connection breaking under load, arcing between windings or to the core, or a tap changer fault, and the transformer should be taken out of service and investigated.
Can transformer oil be replaced or cleaned?
Yes. Oil that is wet or dirty is filtered and dried on site with a vacuum dehydrator; oil that has oxidized (high acidity, low interfacial tension) is reclaimed by passing it through Fuller’s earth or similar, which restores it to near-new condition; or it’s drained and replaced. What none of these fixes is the paper insulation: most of the water in a wet transformer is in the paper, not the oil, so a wet unit is dried out (by hot oil circulation or vacuum) rather than just refilled.
What is FR3 fluid?
A natural ester dielectric fluid made from vegetable oil, sold by Cargill as Envirotemp FR3 and now the standard less-flammable fluid for pad-mounts and indoor units. Its fire point is above 300 °C, against about 160 °C for mineral oil, so FR3-filled transformers are allowed indoors and closer to buildings (NEC 450.23); it’s biodegradable and non-toxic; and it holds far more water than mineral oil, which helps dry the paper. A unit filled with it carries a K-class cooling designation (KNAN rather than ONAN) on its nameplate.
How do I know if a transformer has PCBs?
Look at the nameplate and labels: units built after 1979 say “non-PCB” or give the fluid type, and many sellers certify a test result (under 1 ppm, or under 50 ppm). For a unit built before the mid-1980s with no certification, have the oil tested (ASTM D4059, about $50 to $100). The EPA’s classes under 40 CFR 761: under 50 ppm is non-PCB; 50 to 499 ppm is PCB-contaminated; 500 ppm and above is a PCB transformer, with rules on where it may be used and how it must be disposed of.
Sources: IEEE C57.106 (acceptance and maintenance of insulating mineral oil); IEEE C57.104 (interpretation of gases generated in mineral-oil-immersed transformers); IEEE C57.147 (natural ester fluids); ASTM D3487, D877, D1816, D1533, D974, D971, D1500, D924, D3612 and D4059; EPA 40 CFR 761 (PCBs).